Durnin, Miceli, and Eberly of the University of Rochester have demonstrated, both theoretically and experimentally, the existence of diffraction-free modes in free space laser beam propagation. See Durnin, J., Miceli, J. J., Jr., and Eberly, J. H., Phys. Rev. Lett. 58, 1499-1501 (1987). By passing an ordinary, Gaussian profile laser beam through a thin annular slit and collimating the result, the workers generated a beam with a J.sub.o -type profile. ("J.sub.o " refers to the Bessel function of zeroth order.) The advantage of the J.sub.o -profile over the Gaussian profile is that the former does not suffer geometric divergence (herein referred to as diffraction) as it propagates in free space, with no boundaries nor guiding surfaces present. Of course, plane waves also share this property. However, the J.sub.o -profile has the advantage that the beam's intensity is the greatest at the center, an important feature for laser welding, laser neutral-particle beam guidance.
Unfortunately, in the process of "reforming" the beam's profile, much of the energy is lost in absorption by the mask containing the annular slit itself. This reality severely limits the usefulness of the technique for high power applications. Additionally, although the beam does not undergo diffractive spreading, the radius of the collimating lens determines an effective maximum range. However, this range was shown by Durnin, Miceli and Eberly to usually be much greater than that of the ordinary, collimated Gaussian beam. These researchers also experimentally demonstrated that the J.sub.o -profiled beam had a greatly increased propagation range as compared to the associated Gaussian-profiled beam.
The practical difficulty with employment of this Gaussian-to-J.sub.o coupling mechanism to problems, for instance, related to high energy, long distance laser beam transmission lies in the fact that about 99% of the laser beam's energy is absorbed by the mask; only a fraction is transmitted through the annular slit.
Some of the system(s) detailed below incorporate nonlinear optical mechanisms, including stimulated Brillouin scattering (SBS), two-wave mixing (twm) and degenerate four-wave mixing. Both SBS and twm are described in some detail in Zel'dovich, B.Ya., Pilipetsky, N. F., and Shkunov, V. V., Principles of Phase Conjugation, Springer-Verlag, 1985. Degenerate four-wave mixing (dfwm) is described in U.S. Pat. No. 4,145,671 to Hellwarth as well as in the Zel'dovich et al. text. Both SBS and dfwm are often referred to as optical phase conjugation (opc) processes. Other systems described herein make use of holograms (rather than non-linear media) in the Gaussian-to-Jo coupling.